The Sewer Is a Popular Place
by Saru Wolfe
Summary: Honestly, it wasn't like Mikey meant to bump into government guys on a super secret mission. Of course, he wasn't exactly what the BPRD was expecting, either. TMNT/HB crossover.
1. Cosmic Coincidence

**Disclaimer: Because everyone knows that the people who own big series like TMNT and Hellboy spend their time writing fanfiction.**

**Thanks to RangerKimmy for encouraging this undoubtedly unhealthy enthusiasm for TMNT. Enjoy!**

"Now where is that sucker?"

Michelangelo froze in place, clutching his skateboard as if he intended to use it as a weapon (which he probably could). That definitely didn't sound like anyone who had come down here before, and that list had grown substantially of late.

"I have no idea." This voice managed to be militaristically level and convey annoyance at the same time.

"You'd think a giant turtle wouldn't be that hard to find," the first voice grumbled.

The giant turtle rocked back on his heels and shifted his grip on the skateboard in alarm. He was tempted to show them exactly how hard it could be to find him, but the continuation of their conversation held him there. He might as well figure out what kind of evil ninjas were invading this time. He edged _very_ stealthily toward the bend in the tunnel and leaned around it.

"This looks likely," someone else said. Mikey couldn't tell who had spoken. Five people stared off into a connecting tunnel, and he could hear the sound of rushing water. Only one of them was really visible from the light from the gutter; he was wearing a pressed suit, sunglasses, and a bland expression. Okay, so maybe they weren't ninjas. Ninjas didn't look like they were straight out of a James Bond movie.

"This your stop, Blue?" the first voice grunted.

After a pause, a mild, modulated voice, presumably Blue, said, "Nothing. If it was ever here, it's long gone."

"All right. Who's ready to call it a night?"

"Wait," Blue interrupted softly. "We're not alone."

Mikey backed away slowly. He didn't really start running until he heard multiple footsteps following him, far too close for comfort. He used every trick he knew to lose them while sprinting at full speed, but every time he glanced back, they were still in sight. Those CIA guys were _fast_. Mikey needed a new plan.

"Cut him off!"

A few seconds later, when two agents jumped out in front of him, he sent his skateboard rocketing at one and knocked the other one on the head with his own gun. He only slowed down long enough to grab his skateboard before dashing off to the left. The familiar sound of water gave him an idea. He turned around so that he was jogging backwards and made a rude face at the remaining agents before pushing off a wall and diving into a connecting tunnel. He landed with a splash in the flowing sewage water. "Suckers," he crowed, and began to swim home.

This was new. Mikey had never run into (from) CIA guys before. It was pretty exciting. Maybe if they came after him more often, he'd be able to see some cool spyware in action.

It was this train of thought that prevented Mikey from noticing sooner that someone was following him. Even creepier was the fact that the agent dude was swimming faster than he was. He knew that the skateboard was hindering him, but he was a _turtle_, for crying out loud! What kind of training did spies have to go through, anyway? He set his board up on the concrete and shoved off, moving farther with more powerful strokes. It didn't do any good; the CIA guy was still gaining. It occurred to Mikey that leading a spy straight to his family was a bad idea. As long as it was just the one guy, Mikey could take him and get home before the backup caught up with him.

With this new plan of action, Mikey swung himself up onto the walkway, drew his nunchaku, and waited, bouncing on the balls of his feet and swinging his nunchaku in dizzying loops. He didn't have to wait long. His pursuer had been right behind him. Mikey tensed as a figure dragged itself out of the water, but then his eyes widened. "Dude," he said, drawing out the word to communicate shock and bewilderment simultaneously.

Enormous black eyes not-quite-blinked at him as the being cocked his head curiously, revealing gills on the side of his neck. He slowly stretched one hand toward Mikey, spreading his webbed fingers. Blue skin of varying shades glistened in the dim light. Oh. That would make him –

"Yes," the fish agent said in answer to his thoughts, amusement coloring the calm voice, "I'm 'Blue.'"

Mikey couldn't do anything but stare for a moment, twirling one of his weapons uneasily. This guy didn't look like he wanted a fight. What was going on? Mikey had lost track of the situation. He decided to stick with the safest question for now. "What do you want?"

Blue lowered his hand. "I am from the Bureau of Paranormal Research and Defense. We are hunting a demon which has recently either broken loose or been summoned. It was last reported here, in the sewers of New York City."

"Oh." That kind of made sense. They had been hearing weird, distant noises for the past couple of days, but they hadn't actually found anything. Whoops – that reminded him. He totally wasn't supposed to be out here skateboarding, much less being chased around by agents. With CIA guys involved, there was no way he was going to get out of this one. He was in _so_ much trouble. He might as well drag this out as long as he could. "What kind of demon?"

"It's called Reiki. It is an obscure demon from Japan which, incidentally, resembles a turtle." Blue added wryly, "I apologize for the confusion."

"It's cool." Mikey grinned. "It's my first time getting chased by super spy CIA dudes."

Blue looked dubious, but before he could respond, something on his belt beeped. He held it up to his ear and listened for a moment. "Actually, no. Apparently, we got the wrong giant turtle." He paused. "I don't know, Red. Maybe he can help. Oh, and leave Harris and his men up there, would you?" He hung up. "Red will be here soon. You'll hear him coming," he informed Mikey.

The last ten seconds had reminded Mikey that he had backup, too, and he was starting to get nervous enough to use it. He tucked one of his nunchaku into his belt and pulled out his Shell Cell. His dive into the sewer water had shot it, but a few sharp collisions with the concrete wall had it functioning again. Taking a deep breath, he called, but he held it away from his ear just in case. Sure enough, the first thing out of the speaker was Raphael bellowing, "Where are you?!"

"Uh…" Mikey looked around. "Under 32nd?"

There came the distinct sound of two people wrestling over one phone. "Hurry up and come home," Don said over Raph's, "Get your butt over here!" in the background. "Leo's having an older brother fit, and I don't even think he's the one you'll have to worry about."

Mikey grimaced. "Well… I may have run into some trouble…."

The scuffling noise came again. "What kind of trouble?" Raph growled.

"Trouble?" Leo repeated worriedly in the background.

"What's going on?" Don demanded.

"Geez!" Mikey huffed out. "Chill! You know those weird noises we heard yesterday?" General sounds of assent came from the speaker. "Turns out that there's some monster thing running around, and there are some people down here looking for it."

"People?" Leo asked suspiciously.

"Well…" Mikey began, eying Blue.

Blue cleared his throat politely. "I was wondering whether you could provide us with any information."

"Oh. They want our help," Mikey told his brothers.

Indistinct conversation made it clear that they were discussing this without him. A few seconds later, they all fell quiet at once and a single person took the phone. "Michelangelo?"

At that tone of voice, Mikey automatically tried to shrink in on himself a bit, even though the exchange was over the phone. "Yes, Sensei?"

"I made it clear that none of you were to wander the sewers until we discovered the source of those sounds."

Mikey looked down and fiddled with his nunchaku. "Yes, Sensei."

"We will discuss this further when you are home." The reprimand was over for now, and Master Splinter's voice became much lighter as he continued, "Now, who wants our help?"

"Um…"

"The Bureau of Paranormal Research and Defense," Blue said helpfully, watching his end of the tunnel.

"Yeah… some government thing." Mikey considered. "I think they're all right, though." A thump and a few curses from up the tunnel distracted him from whatever Master Splinter said next. A moment later, something came up behind Blue. Mikey could only assume that the gigantic figure with a stone hand and stubs of horns was Red. Mikey waved at him.

"Michelangelo?"

"Huh?" He returned his attention to the phone. "Yes?"

"I asked whether you are certain that these people will not harm us because of what we are."

Mikey glanced over at the huge red demon in a trench coat and the blue fish guy. "Oh, yeah. I'm sure."


	2. Joining Forces

The only thing Donatello was sure of was that he was not the only one giving Master Splinter a perplexed stare. Master Splinter pretended not to notice as he handed Don's Shell Cell back. Don absently tucked the phone into his belt, trying to comprehend the situation.

Leonardo beat him to it. "Mikey's bringing government agents here?" he asked uncertainly, as if he hoped that he had interpreted Splinter's end of the conversation completely inaccurately.

"Two, if I understood correctly," Splinter replied. "Do not worry, my sons. They will be no threat to us."

Don really hated to be the one to vocalize the next question, but no one else was going to do it. "And… we're trusting _Mikey's_ judgment on this?" he asked incredulously.

"Yes. And mine," Splinter said evenly, giving him a sharp look. No one raised any more arguments after that, though Raph heaved a frustrated sigh that spoke for them all. Don resigned himself to his fate and sat down on the couch. After a moment, Raph leaned against the arm of the couch beside him and crossed his arms, scowling. Leo tried not to pace with mixed results. Master Splinter turned on the television, seemingly oblivious.

A tense ten minutes later, they heard the door opening. The three turtles didn't jump at the noise, but they did shift almost imperceptibly to make their weapons easier to reach.

"Hey, guys!" Michelangelo called cheerily as he poked his head into the room. He received a few glares, but the tension lessened considerably.

Raph stood up. "'Bout time," he grumbled.

"Indeed," Splinter agreed.

Mikey chose to ignore that. "I brought home secret agents," he announced. He disappeared for a moment, and then returned, leading in two beings who were definitely not included in the general CIA stereotype. Don's eyes widened as he took in the odd pair before him. The smaller of the two, he observed, seemed to be a perfect blend of human and ichthyic DNA, much as he and his brothers appeared to be human-turtle hybrids. The other agent, though, was truly an enigma. Don had no idea what kind of genetic alteration could cause red skin or those strange bone structures on his forehead.

Engrossed in study, Don belatedly realized that Mikey had been introducing everyone, and Don was now being pointed at. "This is Donnie," Mikey told the agents.

Don smiled a little sheepishly. "Hi."

Mikey grinned. "And the moody emo turtle over there is Raph."

Hopefully, it was clear that Raph's heated glower was directed at Mikey, not at the guests. "So who're they?" Raph asked.

"Uh…"

The blue agent rescued him. "We are from the Bureau of Paranormal Research and Defense. My name is Abraham Sapien; call me Abe. Nice to meet you all."

"Hellboy," his colleague rumbled.

"Welcome to our home," Splinter said graciously, bowing. "Michelangelo. Ten flips."

Mikey had hoped that the appearance of his new friends would make Master Splinter forget about that. "Aw, man…." With a dramatic, long-suffering sigh, he trudged over to the nearest open space and began. Don didn't pay attention – he had seen Mikey punished with ten flips so many times in his life that he hardly noticed if it happened anymore – but the agents of the Bureau of Paranormal Research and Defense seemed to find it interesting.

"You wanted our assistance, agents?" Splinter prompted, effectively drawing their attention away from Michelangelo.

"Yeah," Hellboy confirmed. "You guys seen any _other_ giant turtles down here?"

Don exchanged puzzled looks with his brothers. "Nope," Raph replied succinctly.

"Have there been any unusual disturbances?" Abe persisted.

"You mean besides Mikey's fascination with the toaster?" Don muttered. Raph snickered.

Leo took charge. "There have been some strange noises coming from farther down in the sewers. They started recently. We went down to check it out, but…"

Mikey, done with ten flips, jumped into the conversation – literally. "But we rushed down there, all geared up and ready to kick monster butt, and there was nothing there!"

"We didn't find anything," Leo concluded reluctantly, discreetly stepping on Mikey's foot. Mikey let out a very indiscreet, "Ow!"

Don perked up. "That's not exactly true," he argued.

"Well, we didn't find anything _useful_," Mikey conceded.

Don barely spared him a halfhearted glare before dashing down the stairs to his lab. He had known that saving those samples would be worthwhile. He sifted through sets of stored vials before finally withdrawing one. It was empty but for a few chips of rock huddled at the bottom. He held it up and tapped it, breaking the fragments loose, and set it in a slot at his work space. He sat in the worn chair and began assembling the necessary apparatus.

He didn't have to look behind him to know that everyone had followed him in. Hellboy certainly made a lot of noise, and so did Mikey's mouth. "What is that?" Abe inquired from behind him.

Don plugged up the Bunsen burner and selected a clean sample plate. "We didn't find a monster, but we found where he had been." He paused to inspect the label on a beaker.

"How did you know he had been there?"

"Big gash in the wall." He reached for his chemical cabinet, but before he could start rolling the chair, Abraham handed him three bottles – the exact bottles he had planned to grab. Don turned around, staring. "How…?"

Abe shrugged. "It comes with being a fellow scientist – and a psychic."

Don blinked turned back to his work station. "Um, thanks. Anyway, some of the cement showed up differently on initial scans, so I gathered some samples to analyze later. Of course," he added darkly, "I've been too busy with repairs to actually analyze them until now."

No one made any comment to that.

Don snapped on his goggles, separated out the sample, and began testing it. He scanned some in the computer, heated part of it on the Bunsen burner, and mixed the rest with selected chemicals. Half an hour later, he was adding all his findings to the results of the computer's scan. He set a few parameters and pressed 'enter.' A loading bar appeared and did _not_ zoom straight to one hundred percent. Don blinked, removing his goggles. At the rate the blue bar was crawling across the screen, one would think it was working through a six-disc RPG rather than an everyday chemical analysis. Brow furrowed, he mentally reviewed the results. If they hadn't been so odd, he probably could have figured out what kind of chemical they were dealing with by now. He drummed his fingers on the desk in thought.

"Got any ideas?" Hellboy asked.

Don jumped; he had rather forgotten that there was anyone else in the room, much less six beings. He cleared his throat, reluctant to admit that he was lost. "Well…" Before he could come up with anything, the computer beeped, demanding his attention. He stared at the data covering the screen. It was full of question marks. That couldn't be good. He clicked frantically on the visual model, enlarging it. The collection of bonded atoms rotated around mockingly, giving him a full view of something far outside his knowledge. Steeling himself, he spun the chair around to face his expectant audience.

"I have never seen anything like that compound in my life," he admitted.

Leo and Raph gawked at him. Mikey looked a little as though he had been told that the Shredder was now writing children's books to raise money for charity and doing anti-drug speeches at elementary schools. "You mean… you don't _know_ what it is?" he asked timidly, eyes round.

Don shook his head shamefully.

"I believe I could help you," Abe offered courteously. He stepped forward and examined the screen, cocking his head to one side. Don watched him. "We have encountered something like this before. I don't know what it is, either," he added apologetically. "But it has always appeared when certain demons are summoned."

"The alien ones," Hellboy added helpfully.

"Namely, demons from another dimension not directly connected to this one," Abe translated.

Raph sighed, annoyed. "What're we looking for?" he asked bluntly.

"Reiki, a turtle demon from Japan. It's mostly black and about…"

Something clicked. "Wait a minute," Don interrupted. "How can it be Japanese if it's from another dimension?"

Abe hesitated and looked at Hellboy. "It's not Reiki at all," he realized.

"So what is it?" Leo pressed, exasperated.

Abe shook his head. "I don't know. I would have to do more research."

Raph flipped out a sai, patience obviously expired. "Well, we'll never know unless we go find out, so let's go find it and do our thing, okay?!"

"Finally, someone who speaks my language!" Hellboy muttered appreciatively.

"Yes!" Mikey did a standing flip as if it hadn't just been used for punishment. "Monster-fighting teams joining forces! Totally awesome!" He bounced up and down. "Can we go now?"

Don tried and failed to restrain a grin at his brothers' antics. "It's probably the most effective approach."

Leo nodded. "Let's go." There were cheers all around. As he gathered a few basic scanners, Don had to admit that he had never seen an odder team of monster hunters.


End file.
